The Red Hills Desert Garden, meant to be a sanctuary for some of southwestern Utah’s endangered native fish, has been polluted by illegally-introduced goldfish and other popular pet species, authorities say.

The park, which is built around a 1,150-foot artificial stream, will need to be closed all of next week as biologists work to remove the invasive species, said Steve Meismer, local coordinator for the Virgin River Program.

More than 1,000 goldfish have been removed this year, along with green sunfish, mosquito fish and other species, all of which are reproducing and threatening the local species.

“The illegally-introduced fish outnumber and out-compete our native and endangered fish,” he said. “We’re at a point where the only option is to salvage the desired fish and eliminate the others. It’s an unfortunate situation.”

The treatments are slated to start Aug. 22, with biologists working to catch and safely transplant local species before the stream is drained and any remaining fish are killed off using rotenone, a self-decomposing chemical typically used in such applications.

Meismer said he estimates more than 1,000 unwanted fish remain in the stream, all of which would have originated from fish first introduced by humans.

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A sign urges visitors against putting fish in or taking fish out of the artificial stream at the Red Hills Desert Garden in St. George on Monday, Aug. 15, 2016.(Photo: David DeMille/The Spectrum & Daily News)

It’s a common problem across Washington County, where the endangered Virgin River chub and Woundfin, among other species, are often forced to compete with invasive fish brought in from outside the area.

Similar problems have been reported in places like the Washington City Boilers, the Tonaquint Nature Center, in ponds and streams up and down the Virgin River and its tributaries.

Last year officials closed Gunlock Reservoir to remove an overabundance of smallmouth bass.

Meismer said he doubts people introducing invasive fish are doing anything malicious, surmising that most of the fish probably started as pets. When people no longer want to take care of them, they look for a place to dump them and head to the nearest body of water, he said.

“It happens all the time,” he said. “I think people just don’t understand the ramifications of dumping a fish somewhere.”

A cooperative effort of multiple agencies, the Red Hills Desert Garden opened last year as a place where residents could view a simulated version of the slow-moving waterways where fish like the woundfin and chub tend to thrive.

Authorities started noticing invasive species living inside the stream almost immediately.

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A group of invasive "mosquito fish" swim in the artificial stream at the Red Hills Desert Garden in St. George on Monday, Aug. 15, 2016.(Photo: David DeMille/The Spectrum & Daily News)

Under the treatment plan, the park would reopen Aug. 26.

“We’re confident this treatment will remove the invasive species and restore a healthy habitat for our native and endangered fish,” said Ron Thompson, general manager of the Washington County Water Conservancy District.

During the treatment, district crews are scheduled to install new signage with the slogan “Don’t ditch a fish” to discourage visitors from creating the same problem all over again.

“Introducing fish, including aquarium species, into ponds, lakes and streams is illegal,” said Rick Fridell, aquatic manager for the Utah Division of Wildlife Resources. “It harms our fisheries, native species and ecosystems. We have a shared responsibility to protect these resources.”